In March 1982 the Federal Communications Commission (FCC) followed the so-called "marketplace approach" to selecting a standard for AM stereo broadcasting and authorized such broadcasting using any of several competing systems. In all of the competing Am stereo systems, the left channel (L) and right channel (R) stereo audio signals are summed to form an L+R signal which is used to amplitude modulate a radio-frequency (RF) carrier in the usual manner.
By subtraction, an L-R signal is also formed which is used in some AM stereo systems to phase modulate (PM) the RF carrier, or, in others to quadrature modulate (QM) the carrier.
The broadcast signals from all AM stereo systems used in the United States are compatible with conventional monaural AM receivers, but are not compatible with each other. That is, an AM stereo receiver designed for one system generally is incompatible with stereo signals broadcast using other systems.
Because of this incompatibility, receiver manufacturers have been faced with the choice of designing stereo receivers for only one AM stereo system, or receivers capable of stereophonic reception of signals of more than one of the systems. Both types of receivers have appeared in the marketplace, and multiple-system receivers have been of two general types. Some have included pilot-signal detection circuits which have provided automatic switching of those receiver circuits required to properly decode the AM stereo signal being received. Other multiple-system receivers have been equipped with manual switches for that purpose.
As used herein the term "co-channel type interference" refers to co-channel interference and interference having similar characteristics, such as might be caused by selective fading or skywave/groundwave interaction.
The degree of annoyance of the adverse effects produced in a multi-system AM stereo receiver by the presence of co-channel type interference is not the same for all AM stereo systems. For example, if the interference occurs when the receiver's AM stereo decoder circuits are operating in the Motorola system mode of reception, which is a phase-separated system, co-channel type interference will produce (in addition to the fluctuations in volume normally experienced in monaural reception) a side-to-side movement of sound sources in the AM stereo image, which movement has come to be known as stereo "platform motion". Under certain conditions of interference, sound sources in the stereo image will swing back and forth, from full left to full right, in synchronism with the co-channel beat frequency, i.e. the difference in carrier frequencies of the desired and interfering signals.
However, when the receiver is operating in the Kahn/Hazeltine system mode of reception, platform motion does not occur because the Kahn/Hazeltine system is an independent sideband (ISB) or frequency-separated AM stereo system. With the Motorola modes of reception, platform motion is an inherent characteristic of the receiver design imposed by the nature of this AM stereo system. In brief, the reasons this system is subject to platform motion, whereas the Kahn/Hazeltine system is not, are as follows.
The Kahn/Hazeltine ISB AM stereo system is essentially a frequency-separated system, i.e. in the transmitted signal, left stereo information is carried essentially single sideband in the lower-frequency sidebands, and right stereo information is carried in the upper-frequency sidebands. On the other hand, in the Motorola system, as well as other phase-separated systems such as the Magnavox system, the stereo information exists in the carrier phase modulation and cannot be separated out by frequency from the double sideband signal. Thus, the Motorola AM stereo system is inherently a phase-separated system and, therefore, is critically sensitive to any interference (such as co-channel) or other disturbances which alter the phase of the broadcast signal before it is decoded in the receiver.
The Kahn/Hazeltine system, on the other hand, being frequency separated, is relatively immune to spurious phase variations in the signal, such as those produced by an undesired co-channel signal interacting with the desired signal in the receiver.
It is, therefore, an object of the invention to provide the capability in an AM stereo receiver to automatically switch the receiver to the Kahn/Hazeltine system mode of reception, if it is not already in that mode, when objectionable co-channel type interference is present, regardless of the type of AM stereo signal being received at the time. If the signal being received is the signal of a phase-separated AM stereo system, such as the Motorola system, the objectionable stereo platform motion which would otherwise be present in the reproduced stereo image will be eliminated by switching the receiver to the Kahn/Hazeltine system mode of reception. While stereo separation will be adversely affected by switching the receiver to the Kahn/Hazeltine mode of reception when receiving a phase-separated stereo system signal, the net effect to the listener will be a significant improvement due to the elimination of platform motion. Furthermore, a "fullness" or ambiance of the sound image will still exist, and it has long been recognized that this "fullness" of sound in a stereo-sound image is of great importance. In fact it has been deemed by some to be of greater importance to the listener than the directional characteristics, as noted in a paper, "The Non-Directional Aspect of Stereo", by C. J. Hirsch and published in the November 1961 issue of the IRE Transactions of Broadcast and Television Receivers, Vol. BTR-7, No. 3, pp. 36-39.
Some prior art AM stereo receivers for phase-separated systems have "solved" the problem caused by co-channel type interference by using a detector which automatically switches the receiver to monaural reception in the presence of co-channel type interference, thereby losing all benefit of stereo reception. However, in contrast with this, the present invention eliminates the platform motion effects of co-channel type interference while retaining desirable ambiance in the reproduced stereo image.
A further object of the invention is to reduce distortion effects which will occur when co-channel type interference is present in an AM receiver. The interaction of the interfering signal with the desired signal in the receiver produces an effect similar to selective fading. Selective fading is defined in the IEEE Standard Dictionary of Electrical and Electronic Terms, ANSI-IEEE Std. 100-1984, Aug. 10, 1984 edition, p. 812, as "Fading that is different at different frequencies in a frequency band occupied by a modulated wave". Consequently, when co-channel type interference is present in an AM receiver, the audio signal recovered by an envelope detector in the receiver will be distorted. The distortion in the output of a synchronous detector under the same interfering conditions will be significantly less. Since it is common practice in receivers for current AM stereo systems to employ envelope detectors to recover the L+R component of the stereo signal, the invention provides automatic switching of the receiver from envelope detection of the L+R signal to synchronous detection when co-channel type interference is present, thereby reducing distortion.